1. Field of the Invention
The present invention relates generally Microwave/RF transmission hardware and more specifically to a coupler that transforms impedance as between its first side port(s) and second side port(s).
2. Description of the Related Art
A 90 degree 3 decibel (dB) hybrid coupler is a passive four-port device. A coupler has four ports: input, transmitted, coupled, and isolated. In a signal-dividing operation, the coupler divides an incident signal (for example, radio frequency (RF) signal or microwave frequency signal) at the input port into two signals paths, one output at the transmitted port and the other output at the coupled port. The signals at the transmitted port and at the coupled port are of equal amplitude and have a relative 90 degree phase difference. As a reciprocal device, the coupler can also combine two signals that are 90 degrees apart in phase. In a combining operation transmitted port and coupled port output that combined signal at the input port. The splitting and recombining properties make the hybrid couplers useful in a wide range of RF circuits, such as low noise amplifier, power amplifier, attenuator, mixers and so on.
Typically, all four ports of the coupler have the same port impedance which is matched to whatever standard RF system impedance is being used in a given system (usually 50 ohms or 75 ohms). The impedance of the circuits in the previous stage and the next stage also need to match to the system impedance, in order to maximize the power transfer through the coupler. For example, in power amplifier design, output matching circuit is essential to transform a typical low impedance of the transistor to the system impedance (usually, 50 ohms or 75 ohms). The impedance transformation is conventionally realized by electrically connecting at least one set of ports of the coupler to extraneous circuit elements. These extraneous circuit elements used to match impedance may be: (i) lumped elements, such as inductors and capacitors, and/or (ii) distributed elements, such as quarter-wavelength transmission lines. As a couple of preliminary notes on terminology: (i) any type of current carrier that acts as a transmission line will generally be herein referred to as a “transmission line; and (ii) a transmission line that has the form of a strip of conductive material will herein generally be called a “strip line” or a “strip transmission line; and (ii) for the most part these terms will be used interchangeably herein, even though they denote a genus (that is, transmission lines) and a species (that is, transmission strip lines). There are inevitably limitations on bandwidth, insertion loss, layout area and cost associated with these extraneous output matching circuits. Generally speaking, the higher the impedance transform ratio that is required, the more performance of the coupler circuitry is degraded.
The difficulty of these matching circuits can be alleviated, if the coupler is structured to have an impedance transforming function. Instead of directly matching to system impedance, a transforming coupler can transform the system impedance into an intermediate impedance first. The matching circuit of the previous or next stage then matches to this intermediate impedance instead of to the system impedance. Choosing the proper intermediate impedance can lower the required impedance transform ratio. This results in the design simplification and/or performance improvement of the matching circuits, as well as size and cost reduction.
As an example of a conventional couple having an impedance transforming function, patent application Publication No: US 2009/0295497 (“497 Dowling”) discloses an impedance transforming hybrid coupler that is realized by cascading two impedance transforming circuits in series with output ports of a conventional hybrid coupler, and integrating these separated function blocks into same package, as shown in 497 Dowling at its FIG. 1. The drawbacks of this solution are increased insertion loss and increased circuit size due to the inserted impedance transforming in the couplers of 497 Dowling.
The following published documents may also include helpful background information: (i) U.S. Pat. No. 6,922,102 (“102 Myer”); (ii) U.S. Pat. No. 7,064,606 (“606 Louis”); (iii) U.S. Pat. No. 7,148,746 (“746 Louis”); and/or (iv) U.S. Pat. No 7,362,170 (“Louis”).
Description Of the Related Art Section Disclaimer: To the extent that specific publications are discussed above in this Description of the Related Art Section, these discussions should not be taken as an admission that the discussed publications (for example, published patents) are prior art for patent law purposes. For example, some or all of the discussed publications may not be sufficiently early in time, may not reflect subject matter developed early enough in time and/or may not be sufficiently enabling so as to amount to prior art for patent law purposes. To the extent that specific publications are discussed above in this Description of the Related Art Section, they are all hereby incorporated by reference into this document in their respective entirety(ies).